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Learning Objectives
After completing this unit students will be able to:
- Explain why files need to be compressed
- Identify metadata and its importance in compressing data
- Demonstrate how images, text, audio, and video can be compressed and decompressed
- Identify and explain “lossy” compression techniques, such as JPEG
- Identify and explain “lossless” compression techniques
- Explain the differences between fixed length encoding and variable length (prefix free) encoding
- Compress/Decompress text documents using dictionaries, Huffman Trees, and LZ77
- Compress/Decompress images using run-length encoding and discarding data
- Compare and contrast two types of video compression: interframe and intraframe
- Explain how psychoacoustics plays a role in compressing audio
Suggested Reading
- Blown to Bits, Chapter 2 – Naked in the Sunlight. pp 19-72
- Blown to Bits, Chapter 3 – Ghost in the Machines. pp 73-108
- Nine Algorithms that Changed the Future, Chapter 7 – Data Compression. pp 105-121
Important Vocab
- Binary Tree – a data structure that can, at most, have two nodes or “branches”
- Bit Depth –refers to the amplitude of the analog wave and specifically to the number of bits used for each sample
- Bit Rate – the number of bits that can be processed per second
- Codec– a computer program that encodes or decodes
- Dictionary– a key in metadataexplaining the instructions to encode or decode compressed data
- Discarding Data – a type of lossy compression that removes unneeded data with no way to get that data back
- Fixed-length Code – blocks of code that are always the same size
- Huffman Tree – a prefix-free binary tree that is the most efficient way to compress individual characters
- Interframe Compression –a video compression that re-uses redundant pixels from one frame to the next, also known as temporal compression
- Intraframe Compression – a technique used by compressing each frame of a video, also known as spatial compression
- Lossless – data compression that does not lose data during compression
- Lossy – data compression that loses data during compression
- Metadata –additional data about the main data, usually at the beginning of a file
- Prefix-Free Code – a specific type of variable-length code that does not use pauses
- Psychoacoustics – a sub-branch of psychophysics that deals specifically with sound
- Psychophysics – a branch of psychology that focuses on the fact that the human eye or ear can not perceive the loss of certain data
- Redundancy – finding frequencies or patterns in code
- Run-Length Encoding – looking for redundancy or patterns as runs in the code
- Sample Rate – how often an analog signal is used when converting to digital, usually measured in kHz
- Uncompressed– all the information from an original file in the same format
- Variable-length Code – each data block can be a different length
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